Within the electronics industry, there is a trend towards automated inspection of circuit boards to detect defects such as missing or misplaced components. By detecting such defects at an early stage of manufacture, preferably prior to soldering of the components to the circuit board, repairs can be performed much more economically. Such "pre-solder" inspection also facilitates better control of circuit board manufacturing.
In U.S. Pat. No. 4,811,410 issued on Mar. 7, 1989 in the name of I. Amir et al. and assigned to AT&T, there is disclosed a technique for inspecting a circuit board by directing a beam of light towards the board at a first angle. The intensity of the light reflected at a second angle from the circuit board is sensed by a linescan-type camera. This type of camera is characterized by a plurality of light-sensing elements arranged in a linear array so that the each element of the camera senses the light intensity from a small region within a thin strip of surface area running across the board surface so as to capture the image of the region. A relative motion is imparted between the circuit board and linescan camera so the camera captures the image of successive strips of area on the circuit board. The output signal of the linescan camera is then processed to retain only the image of those regions of interest in each strip. The images of the regions of interest are then subsequently processed to detect if defects, such as missing and misplaced components, are present.
The linescan inspection system disclosed in the Amir et al. patent (incorporated by reference herein) has proven useful for pre-solder inspection of circuit boards containing through-hole components (i.e., components with leads designed for insertion through openings in the board). The Amir et al. system, especially when operated in the manner taught in co-pending application, Ser. No. 316,004, filed on Feb. 27, 1989, in the name of I. Amir et al. and assigned to AT&T (herein incorporated by reference), has also proven useful for inspecting circuit boards containing surface mount components. Surface mount components differ from their through-hole counterparts in that surface mount components have conductive members (i.e., leads or pads) designed for solder bonding to metallized areas on the surface of the circuit board. Unlike through-hole components, which are commonly wave soldered to the circuit board, surface mount components are solder bonded to the board by a different process. First, solder paste is applied to the metallized areas on the circuit board and then, the conductive members of each component are placed on the paste-coated areas. After component placement, the solder paste is reflowed to bond the conductive members of the component to the circuit board.
While the Amir et al. linescan inspection system can accomplish presolder inspection of circuit boards containing surface mount components, the results are usually less accurate than with circuit boards carrying through-hole components. One reason is that the solder paste applied to the metallized areas on the circuit board tends to be reflective, although much less so than the leads on a leaded surface mount component. In the absence of a component lead, the solder paste thus reflects light into the linescan camera, causing the linescan camera to possibly mistake the paste for a component lead. The ability to distinguish between solder paste-coated metallized areas and the component leads could be enhanced if the presence and nominal position of the components were known, such as by having a three-dimensional image of the circuit board. Further, the ability to obtain a three-dimensional image of the circuit board would also greatly facilitate inspection of the circuit board after application of the solder paste (but before component placement) and after soldering. However, there is no known system which affords three-dimensional imaging using a linescan camera.
Thus, there is need for a technique for accomplishing three-dimensional imaging of a substrate, such as a circuit board, with a linescan camera.